I had done resin beads for the edge using strands of finrope and adding milled fiber to the resin. That gives you a hard edge you can really hone. I use a rounded rail shape with the edge tapering from 1/4" in front of the fins to maybe 1/8" at the tailblock. I think the combo gives me a good compromise between hold and release.

A softer edge, all else equal, will want to slide sideways and will require larger fins. A hard rail in the tail forces water off and away from it, and its through this that you gain control and speed. If you are not pushing against water, somewhere, you are sliding sideways.

I experimented with this myself on a modern shortboard design with very soft, round rails throughout the whole board. It was a quad/thruster option, and even when using 5 large fins, it still would not hold and drive and wanted to slide sideways.

Keep the rails hard behind the front fins, fading to soft about 3-6 inches in front of the front fins.